The present invention relates to a lathe chuck. More particularly this invention concerns such a chuck having two sets of workpiece-holding jaws.
A standard lathe chuck has a chuck body centered on and normally rotated by a headstock about an axis. At least three angularly equispaced jaws are received in this body in respective guides that allow front ends of the jaws to move radially of the axis. Back ends of the jaws are engaged by an actuating plate that is displaced axially in one direction to move the jaws radially inward toward each other and in the opposite axial direction to move them oppositely outward.
In order to machine a brake disk, bearing ring, or the like the initial normally cast workpiece, which is not perfectly rotation symmetrical, must be gripped by the jaws so the workpiece center lies on the axis. The chuck is then rotated so that a workpiece engaging the outer and/or inner surface of the workpiece machines it until it is perfectly centered on the rotation axis.
The chuck jaws normally move perfectly synchronously inward and outward relative to the axis so that they have a tendency to deform a thin-walled workpiece into a more symmetrical shape than it actually has. When the workpiece is machined while thus deformed, it reverts to its original shape when released from the chuck and, therefore, is out of round.
Accordingly it is known to provide three centering jaws on a chuck and a second set of jaws that serve for holding. Due to the multiple points of contact, the likelihood of deforming the workpiece is substantially reduced. The first set of jaws grips the workpiece lightly to center it without deforming it, the second set of jaws is brought in to bear lightly on the workpiece between the first set of jaws, then all the jaws are pressed inward to lock the workpiece in place, normally without deforming it.
While this system works well in theory, in practice there are considerable problems in synchronizing the action of the second set of jaws with the first set. In particular when the jaws are pressing with quite some force it is difficult to avoid deforming the workpiece.
It is therefore an object of the present invention to provide an improved lathe chuck with two sets of workpiece-holding jaws.
Another object is the provision of such an improved lathe chuck with two sets of workpiece-holding jaws which overcomes the above-given disadvantages, that is which can solidly grip an out-of-round workpiece without deforming it.
A lathe chuck has according to the invention a chuck body rotatable about an axis and having a plurality of angularly equispaced and axially forwardly open angled guides, a set of first jaws in some of the guides, and a set of second jaws in the other guides alternating with the first jaws. Respective first and second actuating plates coupled to the respective sets of jaws are axially displaceable to radially displace the jaws in the guides. An actuating shaft is axially displaceable in the body. A rocker connects the shaft with the plates for limited relative axial movement of the plates so that the first jaws can engage the workpiece before the second jaws.
Thus with this system as the actuating shaft is retracted the first jaws, normally numbering three, will engage the workpiece lightly and will center it on the axis. As the actuating shaft continues to retract, the rocker will cause the second jaws to engage the workpiece. With subsequent retraction there will be no more rocking action and all the jaws will move inward identically. Even if the workpiece is out of round, it will be gripped uniformly with no deformation.
The actuating shaft has a tubular front end, a core shaft is axially connected to one of the plates and coaxially received in the actuating-shaft end, and a tube shaft is axially connected to the other of the plates and coaxially surrounds the actuating-shaft end. This actuating-shaft end in turn has a radially throughgoing hole through which the rocker extends. The core and tube shafts have radially aligned seats receiving ends of the rocker. A biasing unit, for instance a spring or pressure-operated piston unit, is engaged between the actuating-shaft end and the tube shaft to bias them apart. With this system the axial tension is effective on the plate of the first jaws to move them synchronously against the workpiece to load the biasing unit, whereupon the second jaws move in to engage the workpiece.
The other plate according to the invention is limitedly rockable on the tube shaft and a plurality of angularly spaced biasing units, springs normally, are engaged between the tube shaft and the other plate. To achieve this effect the other plate has a central seat and the tube shaft has an enlarged head that can swivel in the seat. The other plate can have a planar front face engageable against a planar back face of the one plate so that n a rest position all the jaws are equispaced from the axis.
A plurality of such rockers are angularly equispaced about the axis. Each such rocker has an enlarged central part forming a fulcrum and engaging the actuating-shaft end. The rocker ends are smaller than the central part. To keep the first jaws on center the one plate has an axial forward extension axially slidable in the chuck body.